Investigation of the expansion of the tropical zone and its effect on climate in Thailand

Abstract

Amongst the issues associated with global climate change, the expansion of the tropics has become increasingly important. This expansion is normally demonstrated in the form of changes in the Hadley circulation. The first part of the study will show a series of data sets consisting of brightness temperature from HIRS-UTWV, high cloud cover, precipitation and evaporation from ERA-Interim reanalysis data, deep cloud convection from ISCCP, and aerosol optical depth from MERRA were used to examine the poleward expansion of the Hadley cell in the Greater Mekong Subregion (GMS) over the period 1979 to 2018. A pronounce poleward tropical expansion was found during the winter season but no trends were found for the summer season. Much of the poleward movement appears to be dominated by equatorial convection as measured by high cloud cover. There are some evidences of a shrinking dry zone where precipitation minus evaporation is less than 0. The second part of the study aims to investigate the changes of the Hadley circulation using high cloud cover from the ERA-Interim reanalysis data over the globe. Trends of zonal-mean cloud cover computed over the period 1979-2017 in both hemispheres show that poleward expansion of the Hadley circulation is about 1.6 and 1.2 degree of latitude per 39 years in northern hemisphere and southern hemisphere, respectively. In addition, the expansion for all four seasons was also analysed. The analysis results show that poleward expansion occurs mostly in all seasons and the expansion of the Hadley circulation results in a poleward expansion of the subtropical dry regions. The third part of the study presents the effect of the tropical expansion on climate in Thailand. We calculated the Standardized Precipitation Index (SPI) over Thailand to indicate flood and drought. Then we estimated the correlation between the SPI and tropical expansion indices. The result suggested that the SPI trend is increased in all regions of Thailand and the whole country according to the tropical expansion. That is the tropical expansion induces more convection in the equatorial zone which causes more rain in the Thailand region.

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